As the propane torch is ignited inside the hot air balloon, the volume of the gas increases and the temperature of the gas also increases. This causes the density of the gas to decrease, keeping the pressure constant throughout the balloon, regardless of the small amount of gas lost through the balloon hole.
At high speeds, there is more kinetic energy involved - more energy that will be converted to other forms of energy. This includes breaking a car, or body parts of people involved.
